For light emitting devices, such as light emitting diodes (LEDs), and especially deep ultraviolet light emitting diodes (DUV LEDs), minimizing a dislocation density in the semiconductor layers increases the efficiency of the device. To this extent, several approaches have sought to grow dislocation free semiconductor layers on patterned substrates. Some approaches have proposed various patterning of the underlying substrate. For example, FIGS. 1 and 2 show uses of an overgrowing technique according to the prior art. The technique of FIG. 1 uses patterning of convex protrusions on the underlying substrate and overgrowing a gallium nitride (GaN) semiconductor layer. In the approach of FIG. 2, buildup of semiconductor material in patterned depressions is allowed. A reduction of dislocations may result due to an overall reduction of stress in the semiconductor layer. Another approach uses patterned nanopillars to reduce stress of an epitaxial layer.
Other approaches have used microchannel epitaxy (MCE). FIG. 3 shows an illustration of microchannel epitaxy according to the prior art. In these approaches, a narrow channel is used as a nucleation center containing low defect information from the substrate. An opening in a mask acts as a microchannel, which transfers crystal information to the overgrown layer, while the mask prevents dislocations from transferring to the overgrown layer. As a result, the overgrown layer can become dislocation free. The three-dimensional structure of the MCE also provides another advantage to stress release. The residual stress can be released effectively since the overgrown layer easily deforms. In another approach, a mask is applied at a location of a large concentration of dislocation densities to block their further propagation.
Another approach for controlling dislocations in aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) layers first places seeds including dotted masks on the substrate or a template layer, and then grows the AlN or AlGaN layer over the substrate. The dislocations are attracted towards the center of the seeds and are accumulated there, thereby reducing the dislocation density at other portions of the layers.